1. Field of the Invention
This invention relates to the continuous manufacturer of a titanium or titanium alloy strip in a continuous processing line wherein annealing and pickling, or pickling alone is carried out, and more particularly, to a useful method for making a titanium or titanium alloy strip wherein gloss or glossiness is rendered homogeneous or uniform between different titanium strips or throughout the same titanium strip.
2. Description of the Prior Art
As the development of water front is now in progress and acidic rain is generated, an environment on buildings is beginning to turn severe in recent years. Moreover, a rise in maintenance cost has been experienced. Hence, attention has now been paid to maintenance-free building materials, which are excellent in corrosion resistance and long in lifetime. Under these circumstances, building materials consisting of titanium strips and titanium alloy strips (which may be sometimes referred generically as "titanium strip or strips") are recently used in steadily increasing amounts.
The main reason for this is that titanium has very good corrosion resistance and undergoes little corrosion in an atmospheric environment. Besides, one of factors, with which titanium is favored, is that it assumes a peculiar silver white color and various kinds of design properties can be imparted thereto by surface treatments. As titanium is increasing in its applications to buildings, diverse demands are created for surface design properties, and particularly, it is frequently required to appropriately control a surface gloss.
It is known that the surface finishing method of a pure titanium strip for building material is broadly classified into vacuum annealing finish and pickling finish. The titanium strip obtained by the vacuum annealing finish has an as-rolled surface texture and has a glittering feel, with a loss of a decorative appearance. In addition, gloss control is difficult, which often leads to the problem that a difference in gloss appears between the sites of an actual titanium strip article. In contrast to the vacuum annealing finish, the pickling finish is sometimes favored due to the fact that a surface gloss can be at a relatively low level, and thus, reflection of light can be suppressed as desired.
Where a titanium strip used as a pickling finish material is industrially obtained, usual practice is to use a continuous annealing and pickling line having such an arrangement as shown in FIG. 1. In this line, an atmospheric annealing furnace, a salt bath and a pickling bath are arranged in series, and a titanium strip is passed successively through these devices. As a result, a cold rolled titanium strip for building material can be continuously annealed and de-scaled. Among these steps, the atmospheric annealing is carried out for the purpose of removing the strain caused in the course of cold rolling and controlling the structure in order to obtain desired mechanical properties.
The immersion in the salt bath is for the purpose that the scale formed during the atmospheric annealing is more likely to be removed by pickling. The titanium strip obtained after the atmospheric annealing is immersed in a salt bath containing sodium hydroxide and sodium nitrate as main components at high temperatures of about 500.degree. C., whereupon part of the scale is dissolved in the bath, and simultaneously, the scale is cracked due to thermal shock exerted thereon. Subsequently, pickling using hydrofluoric acid and nitric acid is performed, under which a pickling solution is impregnated in to the base metal through the cracks thereby causing the boundary between the scale and the base metal to be dissolved out. Eventually, the scale can be completely removed to provide metallic luster. Since the continuous annealing and pickling line has such multiple steps as set out above, the relationship between the treating conditions in the respective steps and the gloss has not been always fully made clear. Accordingly, it has been difficult to make a uniform gloss between titanium strips and also on or throughout the same titanium strip although not so difficult as in the case of the vacuum annealing finish. Thus, it is believed that any reliable technique of obtaining such a gloss as required by users has never been established.
For instance, Japanese Laid-open Patent Application No. Hei 8-296071 discloses a method of making a titanium material having a reduced gloss and a good anti-glazing property. However, this method has the problem that gloss cannot be made uniform between different titanium strips or throughout the same titanium strip. Further, in Japanese Laid-open Patent No. Hei 9-3573, there is disclosed a titanium material having good anti-glazing property and its manufacture. However, a uniform gloss cannot be obtained between different titanium strips or throughout the same titanium strip.